To your direct question, yes, I can believe a MTB can be that much slower than a road bike.
On the implicit question of how to transport your telescope, that's a mixed bag. Your MTB with bigger, lower pressure tires probably does a much better job of shock absorption than the road bike's 25 mm tires. Without knowing much about the 'scope, I'd worry that putting it on a rack (or in panniers on a rack) might knock something out of alignment, so the 15 minutes or so you got to your site earlier than with a MTB might be consumed in tweaking the telescope. Better in that instance to keep carrying it on your back, so you get the extra shock absorption of tush plus knees and hips.
Two suggestions. First, see how big a tire you can fit on the TCR. Second, think about getting an extra blinkie and riding all or most of the route on the road, so you don't need to worry about the sidewalk. At least IME, sidewalks seem to be as smooth as the worst (allegedly) paved streets and roads.